The composition of most impervious ceramics is a mixture of naturally-occurring raw materials such as clays, feldspar, kaolins, talcs, etc. In some instances a small amount (e.g., less than 10%) of man-made "body flux", such as frit, glass cullet, etc., is utilized in the composition. The behavior during processing of these impervious ceramic products and the subsequent fired properties of these products are a result of the variations which characterize naturally-occurring raw materials. The variations in naturally-occurring raw materials also cause color variations from run-to-run. This is experienced, for example, in the manufacture of unglazed, impervious, ceramic tile.
As a result, impervious ceramic articles manufactured using naturally-occurring raw materials are often accompanied by a disclaimer which states that run-to-run shade variation is an inherent characteristic of the ceramic article.
The use of naturally-occurring raw material variations also limits the degree to which the dimensional properties of impervious ceramic articles can be controlled.
In the case of impervious ceramic articles prepared by the dust-pressing method, there are principally three different methods of body preparation heretofore used. "Body materials" is a term of art used to describe the raw tile batch. The first and oldest of these methods includes the steps of: wet-grinding a ceramic raw batch, i.e., grinding the raw batch with a sufficient amount of water (such as 30-40 percent by weight) to form a slurry; filtering the slurry; drying the filter cake to a moisture content of about 6-7 weight percent; breaking the cake into small granules; and then pressing the granules to form the desired shape.
In recent years, a more widely-used method of body preparation has been the process known as spray drying. With this process, the raw batch is first either wet ground or dry ground. The ground raw batch is then mixed with a sufficient amount of water (e.g., about 30-40 weight percent) in a blunging operation to form a slurry. The slurry is screened and then sprayed in small droplet form into a hot air drying tower to evaporate most of the water and provide a flowable mass of small granules having a moisture content of about 6-7 weight percent. These small granules are then pressed to form the desired shape.
The third method of body preparation involves "dry mixing" the batch materials and then adding only that amount of water necessary to permit the formation of granules. After granule formation, the granules are passed through a series of screens for particle (granule) sizing. The sized granules are partially dried in a fluid-bed dryer to form granules having a sufficient moisture content for forming the desired shape by pressing. A moisture content of about 6-7 weight percent is typically used.
When naturally-occurring raw materials are used, as is the case with most commercial ceramic articles made under current practices, each of these three body preparation methods has certain limitations.
The properties of impervious ceramic articles are dependent upon the formation of a glassy phase during the firing process, and upon the reactivity of this glassy phase with other body constituents. With naturally-occurring raw materials, the formation of this glassy phase and its desired reaction with other body constituents is best achieved when body preparation involves the very intimate mixing of all body constituents. This is best accomplished when the constituents are wet-milled or blunged, but the energy requirements to evaporate the amount of water required for these processes is extremely high.
Impervious ceramic particles made by the "dry mix" method from naturally-occurring raw materials do not yield the smooth surfaces normally associated with porcelain-type materials. Instead, the surface has a roughness which is less than desirable because it can (and frequently does) trap dirt, etc.
This surface roughness arises from variations in the amount of and the reactivity of the glassy phase formed during firing. In turn, these variations arise, in part, from the fact that, with naturally-occurring raw materials, the degree of intimate mixing required to ensure glassy phase formation and reaction is very difficult to achieve with the "dry mix" process.
While it is not widely practiced, quantities of ground commercial soda-lime glass cullet, in amounts of 60% or less, have been utilized in ceramic body batches in the manufacture of some tile in order to reduce the temperature at which the ceramic articles are fired.
While this practice permits the substitution of a man-made and theoretically compositionally stable batch constituent for one which is naturally-occurring and compositionally variable, it has not permitted the manufacture of ceramic products which have precisely reproducible aesthetic and physical properties.